1. Field of the Invention
The present invention relates to an optical head and an information storage device using the optical head.
2. Description of the Related Art
With the evolution of information society, the amount of information is increasing more and more. In response to such an increasing amount of information, it is desired to develop an information recording method capable of recording information at a very high recording density and a recording/reproducing device based on this information recording method. In an optical disk drive as one kind of information recording/reproducing device, a focused beam diameter related to a recording capacity is limited by the wavelength of light. As measures for increasing the recording density in the optical disk drive, shortening of the wavelength of laser to be used and increasing of the numerical aperture (NA) of an optical lens are known. However, there is a limit to high-density recording due to the diffraction limit. As means for increasing the numerical aperture of an optical lens, there has been proposed a method of using a solid immersion lens to increase the numerical aperture to not less than 1 and utilizing evanescent light leaked from the bottom surface of the solid immersion lens to record information on an optical disk medium. However, this method naturally has a limit to high-density recording because the NA is increased by the refractive index of the solid immersion lens.
As a recording method for realization of high-density recording, attention is focused on a near-field optical recording method such that a minute opening smaller than the wavelength of incident light is formed to utilize near-field light generated from this minute opening, thereby forming a beam spot smaller than the wavelength of the incident light. As a structure (near-field light probe) for generating the near-field light, a pointed optical fiber (optical fiber probe) having a minute opening smaller than the wavelength of light is widely used. Such an optical fiber probe is fabricated by drawing one end of an optical fiber with heat or using chemical etching to thereby point the one end of the optical fiber and next coating the pointed optical fiber except the tip thereof with metal. By introducing light into this optical fiber, near-field light can be generated near the minute opening formed at the tip of the pointed optical fiber. However, this optical fiber probe has a disadvantage such that the light use efficiency is low. For example, when the diameter of the minute opening is 100 nm, the ratio of the intensity of light emerging from the tip of the optical fiber to the intensity of incident light entering the optical fiber is 0.001% or less.
As means for improving the light use efficiency, the following probes have been proposed.
(1) Multistep Pointed Fiber Probe
This probe is an optical fiber probe such that the tapering angle of the tip of the optical fiber is stepwise changed from the root to the tip in two or three steps (Applied Physics Letters, Vol. 68, No. 19, p2612–2614, 1996; Applied Physics letters, Vol. 73, No. 15, p2090–2092, 1998).
(2) Metal Stylus Probe
The stylus of a scanning tunneling microscope (STM) is used as a probe. By irradiating the tip of the stylus with light, intense near-field light is generated near the tip of the stylus (Japanese Patent Laid-open No. Hei 6-137847).
(3) Minute Opening Fiber Probe with Microscopic Metal Ball
This fiber probe is a fiber probe such that a microscopic metal ball is formed at the center of the minute opening at the tip of an optical fiber. Plasmon is excited in the microscopic metal ball by means of light emerging from the minute opening, thereby generating intense near-field light near the metal ball (Japanese Patent Laid-open No. Hei 11-101809).
(4) Glass Piece Probe Coated with Metal
A metal film having a thickness of about 50 nm is formed on a glass piece cut into a triangular prism to excite surface plasmon on the metal film. The surface plasmon propagates toward the apex of the triangular prism to thereby generate intense near-field light near the apex (Physical Review B, Vol. 55, No. 12, p7977–7984, 1997).
(5) Glass Substrate Probe with Metal Scattering Member
This probe is a probe having a glass substrate and a metal scattering member attached to the bottom surface of the glass substrate. Intense near-field light is generated near the metal scattering member (Japanese Patent Laid-open No. Hei 11-250460).
In a near-field optical system, the spacing between a minute structure for generating near-field light and the surface of a sample must be set to several nanometers to tens of nanometers. Accordingly, in the case of using a probe configured by an optical fiber or a glass piece as mentioned above, a special control system is required to control the spacing between the tip of the probe and the sample surface. In general, the spacing is measured by using an interatomic force acting between the probe tip and the sample, and servo control is performed by using a measured value for the spacing. However, in the case of utilizing this servo control, a probe scanning speed is limited because there is a limit to a servo band. Particularly in an optical recording/reproducing device required to have a high data transfer rate, the probe must be scanned over an optical disk at a high speed, and high-frequency spacing variations due to warpage or inclination of the optical disk cannot be controlled by the above servo control method.
To solve this problem, the following probes have been proposed.
(1) Planar opening probe
This probe has a minute opening formed in a silicon substrate by anisotropic etching. Since a peripheral portion about the minute opening is flat, the spacing can be kept constant by pressing the probe on the sample (The Pacific Rim Conference on Lasers and Electro-Optics, WL2, 199).
(2) Opening probe with pad
A quadrangular pyramidal projection having a minute opening at the tip is formed on the bottom surface of a glass substrate, and a pad is formed around the projection. The spacing between the probe tip and the sample can be kept constant by the pad (Japanese Patent Laid-open No. Hei 11-265520).
(3) Surface emitting laser probe with metal minute projection
A metal minute opening and a metal minute projection are formed on the light emitting end surface of a surface emitting laser. Since the minute structure is flat, the spacing can be kept constant by pressing the probe on the sample (Applied Physics, Vol. 68, No. 12, p1380–1383, 1999). It is expected that the light use efficiency can also be improved because of the metal minute projection and the resonance structure.
(4) A patch antenna and a coaxial cable are applied to light, thereby generating near-field light with a high efficiency (Optics Communications Vol. 69, No. 3, 4, p219–224, 1989).
(5) A bow-tie metal piece is used as a minute dipole antenna to thereby generate minute near-field light with a high efficiency (U.S. Pat. No. 5,696,372).
As the performance of an optical memory using near-field light, the following three points are required.    (a) The spacing between the minute structure and the recording medium is controlled precisely on the order much smaller than the wavelength of light.    (b) The beam spot is minute.    (c) The light use efficiency is high, that is, high-speed data transfer is allowed.
The fiber probe having a pointed end whose tapering angle is multi-stepwise changed has a high efficiency 10 to 100 times that of a general fiber probe. However, the efficiency of this fiber probe is yet insufficient in the case of application to an optical recording/reproducing device required to have a light use efficiency of 0.5% or more. Further, since an optical fiber is used, the fiber probe is mechanically brittle and high-speed scanning is therefore impossible. All of the metal stylus probe, the minute opening fiber probe with microscopic metal ball, the glass piece probe coated with metal, and the glass substrate probe with metal scattering member utilize the characteristics of metal to improve the efficiency, and a high light use efficiency can be expected. However, the probe tip in each probe is mechanically brittle in shape, so that each probe is not fit for high-speed scanning. In particular, the metal stylus probe and the glass substrate probe with metal scattering member have a disadvantage such that background light is largely detected because light not applied to the tip of the stylus or the scattering member is also incident on the sample.
As mentioned above, various probes capable of scanning at high speeds have been proposed. However, in the case of the planar opening probe and the opening probe with pad, high-speed scanning is allowed, but the light use efficiency is low. By using a model such that a tapering angle of 30° for the minute opening is formed in an aluminum substrate having a thickness of 560 nm, that the diameter of the minute opening is set to 100 nm, and that light having a wavelength of 400 nm is incident, precise electromagnetic calculation was performed by an FDTD (Finite Difference Time Domain) method. As shown in FIG. 1, in the case that a beam is incident on the metal minute opening having a diameter of 100 nm, the beam diameter in a near-field region just after emergence from the minute opening becomes 160 nm (full width at half maximum) larger than the opening size, and the beam profile has an angular shape especially in the polarization direction of the incident light. Accordingly, high-density recording is difficult.
In the surface emitting laser probe with metal minute projection, it is expected that high-speed scanning is allowed, that the light use efficiency is high, and that the background light is less detected. In the case of generating intense near-field light by using the metal minute projection, the shape of such a metal member must be optimized. However, such shape optimization is not disclosed in Japanese Patent Laid-open No. Hei 11-101809. Further, a manufacturing method for such a metal member is also not disclosed in this publication.
In the method of applying a patch antenna and a coaxial cable to light to thereby generate near-field light with a high efficiency or in the method of using a bow-tie metal piece as a minute dipole antenna to thereby generate minute near-field light with a high efficiency, the light intensity is amplified by using a plasmon resonance condition due to the free electrons of metal. FIG. 2 shows the result of precise electromagnetic calculation by the FDTD method in this case as similar to the above. As apparent from FIG. 2, unless a recording surface is placed within a distance of 2 to 3 nm from the bow-tie antenna plane, the effect of light intensity amplification becomes the same as that in a perfect conductor not using plasmon amplification. That is, the light intensity amplification is not observed and a necessary light quantity of 0.5% or more cannot be obtained. Further, an allowable value for the shape of the bow-tie antenna that can satisfy the plasmon condition is small.
There has been reported a method of increasing a recording density by including a lens-shaped substrate in an optical disk medium (Optical Data Storage 2001 Technical Digest pp277–279, Guerra et al., Apr. 22–25, 2001). The object of this method is to solve the problems on dust, head disk interface, etc., and information is recorded/reproduced to/from a recording film by using light focused by a microlens included in the optical disk medium rather than by using near-field light. The recording density can be increased by increasing the refractive index of the lens material. However, there is a limit to increasing of the refractive index, and the recording density in the circumferential direction of the disk cannot be improved.